Method for carrying out the distillation or reactive distillation of a mixture containing at least one toxic constituent

ABSTRACT

A process is proposed for the distillation or reactive distillation of a mixture that comprises at least one toxic component, the process being carried out in a column containing a structured packing, having at least one packing layer ( 1 ) having a lower end ( 2 ) and an upper end ( 3 ), the packing layer having an internal geometry varying over its height, in such a manner that in the distillation or reactive distillation, in a first, lower region ( 6 ) of the packing layer ( 1 ) a bubbling layer having a predominantly disperse gas phase can be established and simultaneously in a second, upper region ( 7 ) of the packing layer ( 1 ) a film flow having a predominantly continuous gas phase can be established.

[0001] The invention relates to a process for the distillation orreactive distillation of a mixture that comprises at least one toxiccomponent, and to an apparatus for this purpose.

[0002] For heat exchange and mass transfer between liquid and gaseousmedia, in particular for separating mixtures by distillation, platecolumns and packed columns are used in industry. The two types differwith respect to the hydrodynamic operating conditions.

[0003] In plate columns, in each case a bubbling layer forms on theindividual plates where predominantly the liquid is the continuous phaseand the gas is the disperse phase. Between the individual plates arefree spaces in which predominantly the gas is the continuous phase.

[0004] The mode of operation of packed columns differs from platecolumns with respect to hydrodynamics. In this case it is not the liquidbut the gas which forms the continuous phase. The liquid runs as a filmdownward over the packings.

[0005] Structured packings are made up of a multiplicity of individuallayers of packing elements, such as metal sheets, expanded metals andwire fabrics, which are disposed vertically to one another in a regularstructure and are usually held together in a composite by attachmentssuch as metal wires, thin metal rods or metal sheet strips. Usually thepacking elements themselves have a geometric structuring, for example inthe form of folds or circular holes of from about 4 to 6 mm in diameter.The openings act to increase the flood limit of the packing and to makea higher column load possible.

[0006] Examples are packings of the types “Mellapak” , CY and BX fromSulzer AG, CH-8404 Winterthur, or types A3, BSH or B1 from Montz GmbH,D40723 Hilden. The folds of the packing elements of these packings runlinearly and at an angle of from about 30° to 45° to the longitudinalaxis of the packing. The foldings of the packing elements lead to across-channel structure within the structured packing.

[0007] DE-A 196 05 286 describes a special development in which thisangle is further decreased to values of from 3° to 14° in order toreduce the pressure drop of the packings as far as possible in the caseof applications at high vacuum (approximately 1 mbar top pressure).

[0008] In the prior art, structured packings are known which arecatalytically active. A catalytically active distillation packing in aconventional shaping is, for example, the packing “KATAPAK” from SulzerAG, CH-8404 Winterthur.

[0009] Structured packings are usually provided as individual packinglayers which are then arranged in the column stacked one above theother. The packing layers generally have a height of from about 0.17 mto about 0.30 m.

[0010] In the prior art, a structured packing called “Montz” A2 fromMontz GmbH, D-40723 Hilden is known, which has folded packing elementswith curved fold courses. Within a packing element, the gradient ofthese fold courses varies over the height of the packing element. Inthis case the layers of the packing elements alternate so that in eachcase one packing element in which the gradient of the fold line isgreatest at the bottom end of the packing layer is situated next to apacking element in which the gradient of the fold line is greatest atthe top end of the packing layer. The internal geometry of the packinglayer is therefore constant over its height. However, this packing type,in comparison with the usual structured packings, has an unfavorableseparation efficiency.

[0011] Because of the industrial importance of heat exchange and masstransfer processes in chemistry and process engineering, in particularseparation by distillation, a multiplicity of technical developments areaimed at improving heat exchange and mass transfer columns, inparticular distillation columns. Important criteria for an efficienteconomic heat exchange and mass transfer column, in particulardistillation column, are its price, its throughput performance for thegas and liquid stream and the separation efficiency based on the heightof the column. The separation efficiency is usually characterized as thenumber of theoretical plates per meter of column height (n_(th)/m) or asthe height equivalent to a theoretical plate (HETP).

[0012] German patent application 199 36 380.3 (equivalent to NAE19980787), which does not have an earlier priority than the presentapplication, discloses a structured packing for heat exchange and masstransfer that ensures improved throughput and economic efficiency ofheat exchange and mass transfer columns, in which the structured packingis made up with an internal geometry varying over its height so that, inoperation of the packing, in its lower region a bubbling layer having apredominantly disperse gas phase is formed in a targeted manner andsimultaneously in its upper region film flow having a predominantlycontinuous gas phase is formed in a targeted manner.

[0013] In chemical engineering, in many applications mixtures thatcomprise at least one toxic component must be worked up. Examples ofparticularly critical components are processes for preparing andpurifying isocyanates that operate with phosgene as a reactioncomponent, or processes in which prussic acid occurs. For safety reasonsattempts are made to keep the amount of toxic substances as low aspossible.

[0014] It is an object of the present invention, thus to provide aprocess for the distillation or reactive distillation of a mixture thatcomprises at least one toxic component, in which process the amount oftoxic substances in the column is reduced compared with known processesof the same throughput and the same separation efficiency.

[0015] We have found that this object is achieved by means of a processfor the distillation or reactive distillation of a mixture thatcomprises at least one toxic component. The invention features carryingout the process in a column having a structured packing having at leastone packing layer with a lower end and an upper end, the packing layerhaving an internal geometry which varies over its height so that in thedistillation or reactive distillation in a first, lower region of thepacking layer a bubbling layer having a predominantly disperse gas phasecan be established in a targeted manner and simultaneously in a second,upper region of the packing layer film flow having a predominantlycontinuous gas phase can be established in a targeted manner.

[0016] It has surprisingly been found that the amount of toxicsubstances present in a column can be reduced to a considerable extent,that is to say by a factor of about from 2 to 4, if structured packingsare used as are described in German patent application 199 36 380.3,which does not have an earlier priority than the present application.According to the invention the amounts of toxic substances in the columnis minimized by using structured packings having an internal geometryvarying over the height of the packing layers. By the special shaping,the packing can be operated by targeted measures in a region in whichthe liquid in the defined subregions forms the continuous phase and inother subregions forms the disperse phase. It is possible, by suitablechoice of the amounts of liquid and gas, to operate the packing in sucha manner that in the lower region of the packing layer a bubbling layerhaving a disperse gas phase forms in a targeted manner and, in the upperregion of the packing layer a film flow of the liquid having acontinuous gas phase forms in a targeted manner.

[0017] It is known that in packings flooding occurs when the liquidstreams and gas streams exceed certain values. A feature of thetransition to the flooding state with increasing loading is that theliquid which is initially flowing out in disperse form as a film isconverted into a new operating state in which the liquid is thecontinuous phase, similar to a bubble column. A feature of this state ismarked pressure increase and drastic decrease in separation efficiency,since the liquid is back-mixed over a large height region. Theinventively used packing limits this flooded state to a lower subregion.In a further upper subregion, in contrast, the packing is operated asusual, so that the liquid flows out as a film on the packing surface.This subregion further acts as demister.

[0018] These hydrodynamic operating stages described can be achieved, inparticular, by the packing layer being formed having a varyingresistance to flow over its height, the lower region of the packinglayer having a greater resistance to flow than the upper region of thepacking layer. The lower region and the upper region preferably extendin each case over the entire cross-sectional area of the packing layer.

[0019] Preferably, a structured packing is used in which the packinglayer has touching flat packing elements, in particular folded metalsheets, expanded metals, wire fabrics and knitted meshes, the fold linevarying over the height of the packing layer in such a manner that, inthe lower region of the packing layer, it is at a greater angle to thelongitudinal axis of the packing layer than in the upper region of thepacking layer.

[0020] However, it is also possible to form packing elements having afold line which has a curved course, in particular in such a manner thatthe angle between the tangent to the fold line and the longitudinal axisof the packing layer decreases from about 45 to 75°, preferably from 60to 70°, in the lower region of the packing layer, to from 10 to 45°,preferably from 30 to 45°, in the upper region of the packing layer.This curved fold line course is particularly easy and thus economic tofabricate.

[0021] However, it is also possible to provide other courses of thefolds, for example two or more courses which are linear in sections.

[0022] It has surprisingly been found that a further improvement of theseparation efficiency of the packing is possible by constructing theindividual packing layer, as a departure from the previouslyindustrially conventional height of from about 0.18 to 0.30 m, with asmaller height, in particular of from about 0.10 to 0.15 m. In this caselower values from the abovementioned range, in particular for closelypacked packings, that is to say packings having a specific surface areaof from about 500 to 750 m²/m³ are particularly suitable, and incontrast higher values of the packing height are particularly suitablefor coarser packings having a specific surface area of from about 100 to500 M²/m³.

[0023] In a preferred embodiment of the present invention in which thepacking layer has packing elements, at least some of the packingelements are bent over in a tongue-like manner at the lower end and/orat the upper end of the packing layer. Preferably, the packing elementshave cuts for this at the lower end and/or at the upper end of thepacking layer at defined distances which preferably correspond to abouthalf the fold width, so that tongues can be bend over in differentdirections. Particularly preferably, the tongues are bent overalternately toward both sides of the packing element. The depth of thecuts is preferably from 3 to 8 mm.

[0024] The angle which the bent-over tongues make with the packingelement is preferably from about 110 to 150°, so that the tongues areroughly horizontally oriented in the packing layer. The lateralextension of the tongues is chosen so that from about 30 to 60% of theflow cross section is blocked. Preferably, only every second sequentialpacking element is bent over laterally in order to ensure sufficientmechanical stability of the packing layers stacked one above the other.

[0025] In a further preferred embodiment, the packing layer is composedof a combination of at least one first partial packing layer and onesecond partial packing layer, the first and second partial packinglayers differing from one another with respect to their internalgeometries.

[0026] The first packing layer is disposed underneath the second packinglayer. Particularly preferably, the first packing layer and the secondpacking layer are disposed directly one over the other, the firstpacking layer forming the lower partial packing layer and the secondpacking layer forming the upper partial packing layer. The partialpacking layers are preferably designed so that their internal geometrydoes not vary over their height. The first lower partial packing layerpreferably has a height of from 0.02 to 0.10 m, and particularlypreferably from 0.03 to 0.05 m. The second upper partial packing layerpreferably has a height of from 0.05 to 0.2 m, particularly preferablyfrom 0.10 to 0.15 m. The resistance to flow of the first partial packinglayer per meter of height is preferably from about 1.2 to about 5 times,particularly preferably from about 1.5 to about 2.5 times, as high asthe resistance to flow of the second partial packing layer. If thepartial packing layers are composed of packing elements having folds,the resistance to flow of the partial packing layers can be set by theangle which the fold courses or tangents of the fold courses make withthe longitudinal axis of the packing layers. The larger this angle, thehigher the resistance to flow. In the context of the present invention,an embodiment is preferred in which the partial packing layers arecomposed of packing elements having folds, the fold courses or tangentsof the fold courses of the first partial packing layer being at agreater angle to the longitudinal axis of the packing layer than thefold courses or tangents of the fold courses of the second partialpacking layer. Preferred angles have already been mentioned above, whichare here incorporated by reference. The resistance to flow of thepartial packing layers can, furthermore, also be achieved by the size ofthe specific surface area per unit volume.

[0027] Preferably, the partial packing layers have different specificsurface areas per unit volume. Particularly preferably, the first lowerpartial packing layer has a higher specific surface area per unit volumethan the second upper partial packing layer. In this case the specificarea of the first lower partial packing layer is preferably from 20 to100%, particularly preferably from 30 to 60%, greater than that of thesecond upper packing layer.

[0028] The invention also relates to a column for carrying out theinventive process. The column containing structured packings, as aredescribed in the German patent application which does not have apriority earlier than the present application, is used inventively in anembodiment in which the liquid collector and distributor is dispensedwith. It has surprisingly been found that the abovementioned structuredpackings have the advantageous property that they have a certaindistribution action which is completely sufficient for connectionpurposes of redistribution of liquid. As a result the liquid holdup inthe column and the total volume of the column can be further reduced.Only at the top of the column and at the feed point are distributionapparatuses of a simple type, such as ring distributors, provided forthe liquid. Preferably, at least in the lower region of the packinglayer perforated packing materials are used, preferably having aperforation content of from 5 to 50%, particularly preferably from 10 to20%, in order to improve the cross distribution of the liquid in theflooded region.

[0029] In a further preferred embodiment of an inventive column, theamount of toxic substances in the column can be further reduced byintegrating the condenser at the column top into the column. As a resultthe liquid holdup is further reduced. In this measure, recourse can bemade to designs which are customary in distillation technology and havebeen proved in practice.

[0030] The invention is described in more detail below with reference toexamples and a drawing. In the drawing:

[0031]FIG. 1 shows a packing layer 1 of an embodiment of the structuredpacking,

[0032]FIG. 2 shows serially arranged packing elements 4 of a packinglayer 1 of an embodiment of the structured packing,

[0033]FIG. 3 shows serially arranged packing elements 4 of a packinglayer 1 of a further embodiment of a structured packing,

[0034]FIG. 4 shows a section of a packing element 4 of a packing layer 1of an embodiment of a structured packing having laterally bent-overpacking elements 4, in a three-dimensional view,

[0035]FIG. 5 shows serially arranged packing elements 4 of a packinglayer 1 of a further embodiment of the structured packing having thinstrips 15 between the packing elements 4,

[0036]FIG. 6 shows a further embodiment of the structured packing havinga packing layer 1 that is formed from two partial packing layers ofdifferent internal geometry and

[0037]FIG. 7 shows a further embodiment of the structured packing havinga packing layer 1 that is formed from two partial packing layers ofdifferent internal geometry.

[0038] In the figures, the same reference numbers denote the same orequivalent features.

[0039]FIG. 1 shows a packing layer 1 of an embodiment of a structuredpacking according to the present invention. The packing layer 1 has afirst, lower end 2 and a second, upper end 3. It has a height H of, forexample, 0.2 m. The packing layer has mutually contacting flat packingelements 4 made of metal sheets provided with folds (not shown). Thereference number 5 shows the longitudinal axis of the packing layer 1.The packing layer 1, in addition, has a circular cross section. Theinternal geometry of the packing layer 1 varies over its height (notshown). The packing layer 1 has a first, lower region 6, whose internalgeometry differs from a second, upper region 7. The first, lower region6 of the packing layer 1 has a greater resistance to flow than thesecond, upper region 7. By suitable setting of the liquid and gas flowrates, a bubbling layer with a predominantly disperse gas phase forms inthe first, lower region 6 of the packing layer 1 and simultaneously afilm flow of the liquid with a predominantly continuous gas phase formsin the second, upper region 7 of the packing layer. The first, lowerregion 6 of the packing layer 1 and the second, upper region 7 of thepacking layer 1 extend over the entire cross-sectional area of thepacking layer 1. In addition, the first, lower region 6 is directlyjoined to the second, upper region 7. The second, upper region 7 of thepacking layer 1 bounds the second, upper end 3 of the packing layer 1and the first, lower region 6 bounds the first, lower end 2 of thepacking layer 1.

[0040]FIGS. 2 and 3 each show diagrammatically serially arranged packingelements 4 of a packing layer 1 of different embodiments of thestructured packing according to the present invention. The continuouslines show the fold courses of the first, third, fifth etc. packingelement 4 and the dashed lines show the fold courses of the second,fourth, sixth etc. packing element 4.

[0041] The packing elements 4 in FIG. 2 have the same height H of, forexample, 0.2 m, as the packing layer 1. The packing elements 4 consistof metal sheets with folds 8, as a result of which the packing layer 1,which is made up of these packing elements, receives a cross-channelstructure. The folds 8 have a linear course in sections. In the first,lower region 6 of the packing layer 1 the fold courses are at a largerangle α to the longitudinal axis 5 of the packing layer 1 than in thesecond, upper region 7 of the packing layer 1. In the first, lowerregion 6 of the packing layer 1 the fold courses are at an angle α ofabout 60° to the longitudinal axis 5 of the packing layer 1. In thesecond, upper region 7, the fold courses are at an angle α of about 30°to the longitudinal axis 5 of the packing layer 1.

[0042]FIG. 3 shows diagrammatically packing elements 4 of a packinglayer 1 of a further embodiment of the structured packing. The packingelements 4 have folds 8 with continuously curved fold courses. Thepacking elements 4 have the same height H, of for example 0.2 m, as thepacking layer 1. The tangents to the fold courses are, in the first,lower region 6 of the packing layer 1 at a larger angle α to thelongitudinal axis 5 of the packing layer 1 than in the second, upperregion 7 of the packing layer 1. In the first, lower region 6 of thepacking layer 1 the tangents to the fold courses are at an angle of fromabout 45° to about 75° to the longitudinal axis 5 of the packing layer1. In the second, upper region 7, the tangents to the fold courses areat an angle α of from about 10° to about 45° to the longitudinal axis 5of the packing layer. The folds 8 have an approximately paraboliccourse.

[0043]FIG. 4 shows, in three-dimensional view, a detail of a packingelement 4 of a further embodiment of the inventive packing. The packingelement 4, in the detail shown, has folds 8 with a linear course. Thereference number 5 denotes the longitudinal axis of the packing layer 1in which the packing element 4 shown is disposed. At the first, lowerend 2 of the packing layer 1, at distances which correspond to roughlyhalf the fold width, cuts which are from about 3 to 8 mm wide areintroduced into the packing element 4 and tongues 9 are alternately bentover toward both sides so that they are at an angle β of from 110° to150° to the packing element, so that the tongues are roughlyhorizontally oriented in the packing layer. The lateral extension of thetongues is chosen so that from about 30 to 60% of the flow cross sectionis blocked.

[0044]FIG. 5 shows serially arranged packing elements 4 of a packinglayer 1 in a further embodiment of the structured packing. Thecontinuous lines show the fold courses of the first, third, fifth etc.packing element 4 and the dashed lines show the fold courses of thesecond, fourth, etc. packing element 4. The packing elements 4 have thesame height H, of for example 0.2 m, as the packing layer 1. The packingelements 4 have linear folds 8. The reference number 5 designates thelongitudinal axis of the packing layer 1. At the first, lower end 2 ofthe packing layer 1, thin metal sheet strips 15 are disposed between thepacking elements 4. The metal sheet strips are joined directly to thelower end 2 of the packing layer 1. The strips are planar and preferablyhave a height h of from about 15 to 25 mm.

[0045]FIG. 6 shows in longitudinal section a packing layer 1 of anembodiment of the inventive structured packing. The packing layer 1consists of two partial packing layers arranged one above the other, afirst partial packing layer 10 and a second partial packing layer 11.Both partial packing layers 10, 11 together form the height H of thepacking layer 1. The first partial packing layer 10 forms the lowerpartial packing layer and the second partial packing layer 11 forms theupper partial packing layer. The first partial packing layer 10 formsthe first, lower region 6 of the packing layer 1, and the second partialpacking layer 11 forms the second, upper region 7 of the packing layer1. Both partial packing layers consist of a plurality of packingelements 4 which are arranged next to one another or serially. Thepacking elements 4 of the partial packing layers 10, 11 consist of metalsheet and have folds 8 which run linearly. The continuous lines show thefold courses of the first, third, fifth etc. packing element 4 and thedashed lines show the fold courses of the second, fourth, sixth etc.packing element 4. The fold courses are at an angle α to thelongitudinal axis 5 of the packing layer 1, in the first partial packinglayer 10, which is larger than the angle which the fold courses in thesecond partial packing layer 11 are at to the longitudinal axis 5. Inthe first partial packing layer 10, the fold courses are at an angle αof about 60° to the longitudinal axis of the packing layer 1. In thesecond partial packing layer 11 the fold courses are at an angle α ofabout 30° to the longitudinal axis of the packing layer 1. The firstpartial packing layer 10 as a result has a greater resistance to flowthan the second partial packing layer 11. The first partial packinglayer 10 preferably has a height of from 0.02 to 0.10 m, particularlypreferably from 0.03 to 0.05 m.

[0046]FIG. 7 shows, as does FIG. 6, an embodiment of the inventivestructured packing in longitudinal section, with a packing layer 1 whichconsists of two partial packing layers 10, 11. The two embodiments ofFIGS. 6 and 7 essentially agree. The same reference numbers designatethe same parts. Reference is thus made to the comments on FIG. 6. Incontrast to the embodiment in FIG. 6, the fold courses of the presentembodiment of FIG. 7 are at the same angle α to the longitudinal axis 5of the packing layer 1 in the first and second partial packing layers10, 11. However, the lower partial packing layer 10 has a specificsurface area which is greater by 50% than that of the upper partialpacking layer 11. As a result, the resistance to flow is greater in thefirst, lower partial packing layer 10 than in the second, upper partialpacking layer 11.

[0047] By suitably setting the liquid and gas flow rates, in all of theembodiments of the inventive structured packing described in FIGS. 1 to7, a bubbling layer having a predominantly disperse gas phase forms in atargeted manner in the first, lower region 6 of the packing layer 1 and,simultaneously, a film flow of the liquid having a predominantlycontinuous gas phase forms in a targeted manner in the second, upperregion 7 of the packing layer 1.

EXAMPLE

[0048] The experimental column used was a metal column made of stainlesssteel having an internal diameter of 0.1 m and a total height of 6.2 m.It was packed with structured sheet metal packings in cross-channelstructure, the column packings having alternately different specificsurface areas. The packings were each composed of a 0.035 m high packinglayer having a specific surface area of 500 m²/m³ (type Montz B1-500)and a 0.195 m high packing layer disposed above it having a specificsurface area of 250 m²/³ (type Montz B1-250). The folds were at an angleto the longitudinal axis of the column, for both types of packing, ineach case of 45° against the horizontal. Both the low packings and thehigh packings were equipped on their periphery with liquid scrapers madeof wire mesh in order to avoid the liquid being able to pass via therim. The low sheet metal packings had circular perforations of diameter4 mm. In the enrichment part of the column, a total of 0.92 m ofpackings were installed. The stripping part of the column had a packingheight of 2.07 m. A trapping tray was mounted at the feed point of thecolumn. The influent liquid was applied to the trapping tray and passedfrom there to an attached natural circulation evaporator which served asa reboiler. The gas/liquid mixture exiting from this evaporator waspassed to a second trapping tray below the trapping tray. The liquid onthis tray was likewise passed to the evaporator. Excess liquid flowedoff via an inserted overflow pipe. At the bottom of the column a naturalcirculation evaporator was mounted for heating. The vapor was cooled andpartially condensed at the top of the column via two sequentialcondensers which were impinged with cooling water (+22° C.) or brine(−15° C.).

[0049] The feed mixture essentially consisted of the components toluenediisocyanate (TDI) (6.4%), hydrogen chloride (1.1%), phosgene (13.6%),monochlorobenzene (66.4%) and higher-boiling byproducts (12.5%) and werefed into the column in a liquid state at a flow rate of 374 kg/h at atemperature of about 101° C. The column was operated at a pressure of2.65 bar (column top). The heating power of the column bottom was set soas to give a temperature of 166° C. at the bottom of the column. Theheating power of the reboiler in the region of the feed point wascontrolled so that the temperature in the topmost packing layer of thestripping section was 94° C. There was no reflux at the column top.Instead, as reflux liquid, monochlorobenzene was fed in at a feedtemperature of 32° C. at a flow rate of 51.6 kg/h. The overhead productobtained in the first condenser was a stream of 30.2 kg/h having acontent of about 0.9% hydrogen chloride, 41.5% phosgene and 54.8%monochlorobenzene. In the secondary condenser a stream of about 61 kg/hhaving a content of about 22.4% of hydrogen chloride, 75.6% of phosgeneand 1.2% of monochlorobenzene was produced. The bottom product at a flowrate of 334.3 kg/h was highly depleted in hydrogen chloride and phosgeneand only had residual contents of less than 50 ppm of hydrogen chlorideand less than 10 ppm of phosgene.

[0050] The column was operated in the stripping section with a gasloading factor F of 1.7 (Pas)^(0.5). The packings used in theexperimental plant are upscaleable and may be used in a production plantin the same design and loading. As a safety factor, the packing heightwill be increased in a large scale plant from 2.07 to 3 m. In contrast,in comparable production plants currently 15 valve trays must be used inthe stripping section, which occupy a height of 6 m at a tray spacing of0.4 m. The trays are operated at an F factor of about 0.9 (Pas)^(0.5).The volumetric ratio of packings to trays is thus 3/1.7:6/0.9=0.265.Thus, in the gas space of the column internals with packings, that is tosay in the inventive process, only about 26.5% of the toxic substancesare present which are present in the case of trays. If the liquid holdupof the packings of about 5% and of the plates of about 3% is taken intoaccount, this gives, as a further advantage with packings, a reductionin liquid holdup of about 44%, compared with a plate column.

We claim:
 1. A process for the distillation or reactive distillation ofa mixture that comprises at least one toxic component, which comprisescarrying out the process in a column containing a structured packingthat has at least one packing layer (1) having a lower end (2) and anupper end (3), the packing layer having an internal geometry varyingover its height so that in the distillation or reactive distillation ina first, lower region (6) of the packing layer (1) a bubbling layerhaving a predominantly disperse gas phase can be established andsimultaneously in a second, upper region (7) of the packing layer (1) afilm flow having a predominantly continuous gas phase can be establishedin a targeted manner.
 2. A process as claimed in claim 1, wherein thepacking layer (1) has a resistance to flow which varies over its height,the lower region (6) of the packing layer (1) having a greaterresistance to flow than the upper region (7) of the packing layer (1).3. A process as claimed in claim 2, wherein the packing layer (1) hasmutually contacting flat packing elements (4), in particular foldedmetal sheets, expanded metals, wire fabrics or knitted meshes, the foldline (8) being varied over the height (H) of the packing layer (1) insuch a manner that it has a greater angle to the longitudinal axis (5)of the packing layer (1) in the lower region (6) of the packing layer(1) than in the upper region (7) of the packing layer (1).
 4. A processas claimed in claim 3, wherein the fold line has a curved course suchthat the angle between the tangents to the fold line (8) and thelongitudinal axis (5) of the packing layer (1) decreases from about 45to 75°, preferably from 60 to 70° in the lower region (6) of the packinglayer (1) to from 10 to 45°, preferably from 30 to 45°, in the upperregion (7) of the packing layer (1).
 5. A process as claimed in claim 3,wherein the fold line (8) has a course which is linear in sections, thefold line (8) in the lower region (6) of the packing layer (1) being atan angle (5) of the packing layer (1) of from 45 to 75°, preferably from60 to 70°, and the angle of the fold line (8) to the longitudinal axis(5) of the packing layer (1) decreases toward the top in one or moresteps to from 10 to 45°, preferably from 30 to 45°.
 6. A process asclaimed in one of claims 1 to 5, wherein the height of a packing layer(1) is from 0.05 to 0.20 m, preferably from 0.10 to 0.15 m, and theheight of the lower region (6) of the packing layer (1) is from 0.02 to0.1 m, preferably from 0.03 to 0.05 m.
 7. A process as claimed in one ofclaims 1 to 6, wherein the packing layer (1) is composed of acombination of at least one first partial packing layer and one secondpartial packing layer (10, 11), the first partial packing layer (10) andthe second partial packing layer (11) differing with respect to theirinternal geometry, in particular where the resistance to flow of thefirst partial packing layer (10) is from about 1.2 to 5 times,preferably from 1.5 to 2.5 times, as high as the resistance to flow ofthe second partial packing layer (11).
 8. A process as claimed in one ofclaims 1 to 7, wherein the lower region (6) or the first partial packinglayer (10) has a higher specific surface area per unit volume comparedwith the upper region (7) or the second partial packing layer (11), inparticular a specific surface area per unit volume which is greater byfrom 20 to 100%, preferably by from 30 to 60%.
 9. A column for carryingout the process as claimed in one of claims 1 to 8, wherein the columndoes not have a liquid collector or distributor.
 10. A column forcarrying out the process as claimed in one of claims 1 to 8 or asclaimed in claim 9, wherein the condenser at the column top isintegrated into the column.